With the advance in the miniaturization process of semiconductor devices, problems relating to IR-DROP (a drop in voltage) in power supply wiring or the like and SI (Signal Integrity) such as crosstalk have become more significant.
Furthermore, as signal and clock speeds have been elevated, the degree of single-phase signal degradation has become more extreme. For this reason, wide spread use is made of circuits that adopt differential or multiphase arrangements in order to avoid the problems mentioned above. In this case, however, manual routing of wiring is used primarily as the routing method.
FIG. 14 is a diagram illustrating a method of automatically routing differential signals as described in the specification of Japanese Patent Kokai Publication No. JP-A-09-044539 (Patent Document 1). As shown in FIG. 14, a positive-phase output 401a and a negative-phase output 401b of a cell 401 are connected to a positive-phase output 402a and a negative-phase output 402b, respectively, of a cell 402 by wirings 409a and 409b, respectively. After one signal (referred to as “signal A”) of differential wiring has been routed while avoiding a wiring-inhibited area A, a path of the other signal (referred to as “signal B”) is searched for so as to connect the wiring for this signal.
It is assumed in FIG. 14 that the signal A is at 409a and that signal B is at 409b. If, when the search for signal A (409a) is conducted, signal B (409b) engages a wiring-inhibited area B, as shown in FIG. 15, then the signal routing of signal A (409a) is redone during the search for the path of signal B. If routing can be performed with neither signal A nor signal B engaging a wiring-inhibited area, then routing is completed.
[Patent Document 1]
Japanese Patent Kokai Publication No. JP-A-09-044539
Thus, if a routing failure occurs owing to a wiring-inhibited area when signal B is routed after signal A, the signal A must be re-routed. This lengthens the time required for routing.
Further, if differential wiring is used mainly by a high-speed signal and the routing of signal B is performed in a grid adjacent to the wiring of signal A, the capacitance between the two wirings doubles owing to the differential signals and it is very likely that signal degradation will increase.